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A paradoxical pulse, or pulsus paradoxus, is a medical sign where the systolic blood pressure drops by more than \(10\text{\ mm\ Hg}\) during inhalation. The "paradox" is that it's the opposite of the normal physiological response where blood pressure either stays the same or increases slightly upon breathing in. This happens due to pressure changes in and around the heart that restrict its ability to fill with blood during inspiration. It is a sign of underlying heart or lung conditions like cardiac tamponade, constrictive pericarditis, or severe asthma.